Virtual Memory In this lecture, slides from lecture 16 from the course Computer Architecture ECE 201 by Professor Mike Schulte are used with permission.
Virtual Address Space Physical Address Space Virtual Address (from processor) 10 V page no. offset Page Table Page Table Base Reg Access Rights V PA index into page table table located in physical memory 10 P page no. offset Physical Address (main memory) What is virtual memory? • Virtual memory => treat memory as a cache for the disk • Terminology: blocks in this cache are called “Pages” • Typical size of a page: 1K — 8K • Page table maps virtual page numbers to physical frames • “PTE” = Page Table Entry
Virtual Memory • Virtual memory (VM) allows main memory (DRAM) to act like a cache for secondary storage (magnetic disk). • VM address translation a provides a mapping from the virtual address of the processor to the physical address in main memory and secondary storage. • VM provides the following benefits • Allows multiple programs to share the same physical memory • Allows programmers to write code (or compilers to generate code) as though they have a very large amount of main memory • Automatically handles bringing in data from disk • Cache terms vs. VM terms • Cache block => page • Cache Miss => page fault
4 Qs for Virtual Memory • Q1: Where can a block be placed in the upper level? • Miss penalty for virtual memory is very high • Have software determine location of block while accessing disk • Allow blocks to be placed anywhere in memory (fully associative) to reduce miss rate. • Q2: How is a block found if it is in the upper level? • Address divided into page number and page offset • Page table and translation buffer used for address translation • Q3: Which block should be replaced on a miss? • Want to reduce miss rate & can handle in software • Least Recently Used typically used • Q4: What happens on a write? • Writing to disk is very expensive • Use a write-back strategy
Virtual and Physical Addresses • A virtual address consists of a virtual page number and a page offset. • The virtual page number gets translated to a physical page number. • The page offset is not changed 20 bits 12 bits Virtual Page Number Page offset Virtual Address Translation Physical Page Number Page offset Physical Address 18 bits 12 bits
Address Translation withPage Tables • A page table translates a virtual page number into a physical page number. • A page table register indicates the start of the page table. • The virtual page number is used as an index into the page table that contains • The physical page number • A valid bit that indicates if the page is present in main memory • A dirty bit to indicate if the page has been written • Protection information about the page (read only, read/write, etc.) • Since page tables contain a mapping for every virtual page, no tags are required.
Accessing Main Memory or Disk • If the valid bit of the page table is zero, this means that the page is not in main memory. • In this case, a page fault occurs, and the missing page is read in from disk.
Determining Page Table Size • Assume • 32-bit virtual address • 30-bit physical address • 4 KB pages => 12 bit page offset • Each page table entry is one word (4 bytes) • How large is the page table? • Virtual page number = 32 - 12 = 20 bytes • Number of entries = number of pages = 2^20 • Total size = number of entries x bytes/entry = 2^20 x 4 = 4 Mbytes • Each process running needs its own page table • Since page tables are very large, they are almost always stored in main memory, which makes them slow.
miss VA PA Trans- lation Cache Main Memory CPU hit data Caching Virtual Addresses • Virtual memory seems to be really slow: • Must access memory on load/store -- even cache hits! • Worse, if translation not completely in memory, may need to go to disk before hitting in cache! • Solution: Caching! (surprise!) • Keep track of most common translations and place them in a “Translation Lookaside Buffer” (TLB)
Virtual Address Space Physical Memory Space virtual address physical address off off page page 2 0 1 3 frame page 2 2 0 5 Making address translation practical: TLB • Virtual memory => memory acts like a cache for the disk • Page table maps virtual page numbers to physical frames • Translation Look-aside Buffer (TLB) is a cache for translations Page Table TLB
Translation-Lookaside Buffer (TLB)(See Figure 7.24 on page 591) • A TLB acts a a cache for the page table, by storing physical addresses of pages that have been recently accessed.
TLB Characteristics • The following are characteristics of TLBs • TLB size : 32 to 4,096 entries • Block size : 1 or 2 page table entries (4 or 8 bytes each) • Hit time: 0.5 to 1 clock cycle • Miss penalty: 10to 30 clock cycles (go to page table) • Miss rate: 0.01% to 0.1% • Associative : Fully associative or set associative • Write policy : Write back (replace infrequently) • The MIPS R2000 TLB has the following characteristics • TLB size: 64 entries • Block size: 1 entry of 64 bits(20 bit tag, 1 valid bit, 1 dirty bit, several bookkeeping bits) • Hit time: 0.5 clock cycles • Miss penalty: Average of 16 cycles • Associative : Fully associative • Write policy: write back
Example: R3000 pipeline includes TLB stages MIPS R3000 Pipeline Dcd/ Reg Inst Fetch ALU / E.A Memory Write Reg TLB I-Cache RF Operation WB E.A. TLB D-Cache • TLB • 64 entry, on-chip, fully associative, software TLB fault handler Virtual Address Space ASID V. Page Number Offset 6 12 20 0xx User segment (caching based on PT/TLB entry) 100 Kernel physical space, cached 101 Kernel physical space, uncached 11x Kernel virtual space Allows context switching among 64 user processes without TLB flush
Handling TLB Misses and Page Faults • When a TLB miss occurs either • Page is present in memory and update the TLB • occurs if valid bit of page table is set • Page is not present in memory and O.S. gets control to handle a page fault • If a page fault occur, the operating system • Access the page table to determine the physical location of the page on disk • Chooses a physical page to replace - if the replaced page is dirty it is written to disk • Reads a page from disk into the chosen physical page in main memory. • Since the disk access takes so long, another process is typically allowed to run during a page fault.
Virtual Memory Summary • Virtual memory (VM) allows main memory (DRAM) to act like a cache for secondary storage (magnetic disk). • Page tables and TLBS are used to translate the virtual address to a physical address • The large miss penalty of virtual memory leads to different strategies from cache • Fully associative • LRU or LRU approximation • Write-back • Done by software